1. Field of the Invention
The invention relates to digital transmission systems, and particularly, to a higher order digital transmission system including a digital multiplexer having N parallel inputs and a digital demultiplexer having N parallel outputs and a common digital path for transmitting N mutually synchronized digital signal streams between said multiplexer and demultiplexer.
2. Description of the Prior Art
In a digital multiplexer, N incoming signal streams (tributaries) are combined to form one outgoing signal stream, while the opposite operation occurs in the demultiplexer. Multiplexing the incoming signal streams is effected by means of interleaving, which implies that a bit from a binary signal stream 1 is followed by a bit from a binary signal stream 2 etc. The resultant outgoing signal stream has a digital rate which is higher than or equal to N times the digital rate of the N incoming signal streams. This is caused by the fact that the outgoing signal stream requires its own frame word and a few service bits, which are added to the outgoing signal stream. Provisions are also taken to compensate for frequency differences between the tributary signal streams and the multiplex clock. This is necessary since each of the tributary signals has its own free-running clock frequency. To this end, idle bits are injected into the multiplex signal, together with control bits, which indicate the status of the idle bits. This process is called positive justification and in general is the simplest manner of multiplexing plesichronous signal streams.
When digital signals are conveyed over long distances it is customary to use a form of encoding in the transmitter portion and a form of decoding in the receiver portion of the transmission system, such that the digital signal is adapted to the digital path. This digital path may, for example, be constituted by symmetrical or coaxial cables, optical fibers or the air. One of the objects usually is the suppression of the direct current component, which permits of the use of alternating current couplings in the transmission system, and direct current supply of the regenerators from the transmission system via the transmission cable being possible. Another object often is to increase the pulse density of the digital signal to be conveyed, or to ensure a minimum pulse density such that recovering a clock signal, required in regenerative circuits, is possible.
In a block encoding arrangement, use is made of what are commonly referred to as code translation Tables, such as the Tables described in, for example, Philips Telecommunication Review, vol. 34, no. 2, June, 1976, pages 72-86. A series/parallel converter which divides the bit stream applied to its input in consecutive blocks of a predetermined number of bits n is provided at the input of the encoding arrangement. A block of n bits is thereafter converted with the aid of the translation code matrix into a new block of m symbols in accordance with a specific instruction. Blocks of m symbols are reconverted at the output of the encoding arrangement with the aid of a parallel/series converter into a bit stream which is conveyed to the receiver portion of the digital transmission system via the digital path (cable, optical fiber). In the receiver portion of the system, the bit stream applied there is subjected to a reverse process with the aid of the decoding arrangement. Examples of an encoding arrangement and a decoding arrangement are described in, for example, Proceedings 17th International Scientific Congress on Electronics, Rome, 16-18, March 1970, pages 275-283.
A higher order digital multiplex system of the above-defined type is disclosed in, for example, C.C.I.T.T. Recommendation G922. In this disclosure the frame structure of a multiplex system having 4 tributary signal streams of 140 Mbit/s each is described. A frame has a length of 2688 bits and comprises a 12-bit frame synchronizing word; 4 service bits; 4 5-bit justification control words, one for each tributary signal stream; 4 justifiable bits, one for each tributary signal stream and 2648 time slots for the information from the four tributary signal streams. The block encoding arrangement is arranged subsequent to the multiplexer and the block decoding arrangement precedes the demultiplexer. This results in it being necessary that both the block encoding arrangement and the block decoding arrangement must be operated at the full line rate. When this line rate increases to above 565 Mbit/s, realizing the encoding and decoding arrangements becomes problematical, as low-dissipation digital modules are required. In the present state of the art of the industrial integration processes it is not possible to realize these modules, or it is very difficult to do so. Consequently, it is very difficult to apply in a higher order digital transmission system the same design philosophies customary for a lower order digital transmission system.